1. Field
The present invention relates generally to the generation of radiation, and more particularly to systems for delivering such radiation for therapeutic purposes.
2. Description
According to conventional radiation therapy, a radiation beam is directed toward a tumor located within a patient. The radiation beam delivers a predetermined dose of therapeutic radiation to the tumor according to an established therapy plan. The delivered radiation kills cells of the tumor by causing ionizations within the cells. In this regard, radiation therapy systems are designed to maximize radiation delivered to the tumor while minimizing radiation delivered to healthy tissue.
A conventional radiation therapy system utilizes X-radiation energies in excess of 1 MeV. State-of-the-art therapy systems generate this “MegaVolt X-Radiation” (MVR) using linear accelerators. In contrast, tube-based X-ray systems generate “KiloVolt X-Radiation” (KVR) having photon energies roughly between 20 keV to 200 keV. These KVR systems have long have been used for imaging and for other purposes. KVR systems may be much cheaper, simpler and more reliable than the linear accelerators used in MVR systems. Environmental safety is also of less concern with KVR systems, which typically require 3 mm of lead shielding as opposed to the 2 m of concrete shielding required for MVR systems.
Despite the foregoing advantages of KVR, MVR is often preferred for therapeutic use because of the high-energy electrons created by Compton scattering of MVR. Most tissue damage caused by KVR results from photoelectric absorption. Particularly in the case of low-energy KVR (photon energy<20 keV), damage resulting from photoelectric absorption is greatest at the surface of a radiation/tissue interaction and decreases with depth into the tissue. Consequently, a KVR beam of uniform or decreasing flux density (i.e., a divergent beam) may cause greater tissue damage at a patient's skin than at a therapy area within the patient's body.
Several existing techniques attempt to address this drawback of KVR therapy. A KVR therapy system such as those described in U.S. Pat. No. 6,366,801 to Cash et al uses a point radiation source which produces a divergent beam of traditional medical X-rays having energies in the kilovoltage range and focuses the beam on a target using a lens designed for this purpose. By focusing the radiation onto the target, the energy per unit area increases with proximity to the target. As a result, tissue damage at a portion of the target is greater than tissue damage at a same-sized portion of the radiation/skin interaction site. Efforts to increase the target-to-skin dose ratio include the development of lenses for focusing the radiation at greater angles of convergence and/or the injection of radiation-absorbing contrast agents at the target.
Also proposed are methods in which a patient is positioned, a target is irradiated by a radiation beam, the patient is repositioned such that a subsequent radiation beam would intercept an area of the patient's skin that was not irradiated by the previous radiation beam, and the target is irradiated again. The patient may be repositioned and the target irradiated several times. Still other methods include moving the radiation beam so as to scan the target. None of these existing techniques have proved to be satisfactorily efficient and/or effective in providing therapeutic KVR.